Gene expression in two hybrid cell types constructed by nuclear transplantation will be examined. In the first type, mouse fibroblast (A9) nuclei were transplanted into rat hepatoma (HTC) cytoplasts. Analysis indicated that a liver-specific steroid-inducible tyrosine aminotransferase (TAT), apparently mouse in origin, was activated in the hybrid cells. Experiments designed to acquire definitive biochemical evidence that silent mouse genes were indeed activated are in progress. A detailed analysis of early events in the life of these reconstituted cells, including a two-dimensional gel electrophoresis analysis of polypeptide synthesis, will be performed. Also, the mechanism of activation and the nature of the hypothetical inducing substance are being investigated. These studies involve the introduction of various macromolecular fractions into the mouse fibroblasts using several microinjection and transfection techniques. In the second type of hybrid cell, dormant erythrocyte nuclei were transplanted into mouse or chick fibroblast cytoplasts. Synthesis of two forms of alpha chain and an early embryonic beta-family chain were detected in the reconstituted cells. No adult beta globin chain was detected. A detailed analysis of transcription in these cells is being performed, using cloned globin gene family sequences as molecular probes. Both of these hybrid cell systems promise to prove useful in investigations into the relationship of chromatin structure to gene expression.